WHERE TO PLACE A RAIN GARDEN. Look for existing low points in your yard at least 10 feet away from property lines and houses, but near the impervious surface that's causing the runoff. The size of your rain garden will depend on local rainfall, the tributary area and the soil type. Poorly draining soils can be amended with mulch, which functions a bit like a giant sponge, holding water where it can be picked up by plants.
If your new driveway is on a billiards-table-flat part of your property, don't despair. Replace impermeable asphalt with attractive paving made of concrete masonry units with gravel-filled gaps in between. Water seeps through the spaces and into a thick substrata of angular, crushed rock. The voids between the rocks act as a temporary sponge. Slowly the water seeps into the soil below, helping to recharge the water table while sequestering pollutants and allowing soil organisms to neutralize them.
Although the size of this Whole Foods parking lot in Santa Barbara, California, is out of scale for most homes, it illustrates a sound principle for those undertaking new construction. If you create a cross pitch that sends water to a planted island through cuts in the curb, all that nasty stuff that accumulates on your new driveway can be scrubbed clean.
Every little bit counts. Even a little divot like this is a good thing. If space allows, these can be strung throughout your yard as what some stormwater experts call a detention train. Be sure to select plants that don't mind having their toesies wet during the rainy season. Ideally, these will be native plants that occur in boggy areas in nature. Check with your local native plant society.
Depending on your garden's surroundings, something as simple as a dry creek bed might be all that's needed to slow the flow of water, reduce the chances of erosion and give the water time to seep into the soil. In a setting like this forest, a simple palette of native plants that don't mind having their toes wet (think ferns, sedges and reeds) will appear right at home.
Mini rain gardens can be situated at the low side of a yard to filter excess runoff from the lawn. Even if you've gone organic in your lawn care regimen, there's still the problem of nutrients (nitrogen, phosphorus, potassium and other elements) entering waterways. They don't just make your lawn grow; they can accelerate algae growth, which in turn upsets the habitat of fish and other aquatic life.
This design makes the most of a narrow yard by including an intimate chill-out space, an alluring display of low plants and open space where water can flow and infiltrate the soil. Unlike concrete or mortared brick, which shed water, flagstone laid with generous gaps like this gives rainwater a chance to move directly into the ground.
The cottage has a bedroom and a full bathroom inside; its living and dining rooms are outside. The roof shades the patio in summer, and the fireplace warms chilly nights.
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